1. Technical Field
The present invention relates to linear integrated circuits, designed primarily for consumer products where a battery is used to supply backup or operating power. More particularly the present invention provides a means of obtaining low level bias current without the use of high value voltage dropping resistors in the integrated circuit.
2. Background Art
It is frequently necessary to provide a bias current for an integrated circuit, of a few microamps from a relatively high voltage source such as a 9 volt battery, without utilizing high value voltage dropping resistors. For example, in a bi-polar integrated circuit resistor values above approximately 10,000 Ohms are costly in terms of area required and possible poor stability characteristics. At the same time the only available source of power may be a battery of relatively high voltage. It therefore becomes necessary to obtain the bias current by means of some circuitry which will not require the aforementioned high value resistors.
Since transistors are more economical of space it makes sense to use transistors as a method of providing the required current. One method would be to use a conventional current source design. A current source circuit of the prior art usually requires a reference current and therefore is wasteful of battery power in the event a battery source is used. A more desirable circuit is one which is self starting (boot-strapping) and does not "use" any current other than what is "delivered" to the load circuit. Another requirement of the circuit is that it have reasonable temperature stability so that at most only one order of magnitude of current change occurs over a temperature range for example of 0 degrees to 50 degrees Centigrade.
This degree of preciseness of the current is not a requirement since the circuit merely has to produce a certain minimum of current, e.g. 0.2 microamps at the worst case temperature (0.degree. C.) for allowing start up of an operating circuit when power first becomes available. Such circuitry is allowed to have as much as 15 microamps at the worst case or high temperature e.g. 50.degree. C. The invention described herein meets such requirements.